This invention relates to thickness measurement and more particularly to such measurement utilizing coherent microwave radiation.
It is often desirable, or even necessary, to determine the thickness of materials when their surfaces are not readily available or are located in hostile environments. For example, high temperature furnaces are lined with refractory materials whose integrity is critical for continued operation of the furnace. Such high temperature furnaces are used in many industries such as the manufacture of materials such as glass and steel, in the remediation of wastes, and for power production. Thus, the refractory insulation lining in such furnaces is subject to the harsh furnace conditions which can cause it to deteriorate. The lifetime of a furnace, which in most cases represents a multi-million dollar capital investment, is determined by the condition of this refractory lining. In addition, the productivity and scheduling of furnace facility operations is determined in part by requirements for minimizing refractory wear, or refractory inspections, and for refurbishment.
It is important to be able to monitor the condition of the refractory material while the furnace is in operation. Such real-time monitoring capability improves the productivity and lifetime of furnace facilities. Unnecessary furnace downtime for refractory inspections can be minimized and furnace operations which cause rapid refractory deterioration can be quickly corrected before significant damage is done. In addition, with real-time monitoring, timely corrective action can be taken once the refractory deterioration has reached a critical stage so as to prevent a catastrophic furnace failure. It is also desirable to monitor refractory condition without needing access to the interior of the furnace because of its hostile environment.
A number of methods and instruments have been developed heretofore and used for refractory lining thickness measurements. All of the known existing methods and devices require either access into the furnace to view internal exposed surfaces or special modifications in the furnace lining itself such as, for example, to embed instruments or to add special materials such as radioactive tracers during furnace lining construction. Other techniques involve indirect methods which rely on mathematical temperature models. No known prior art technique functions by simply viewing the outside of the refractory lining.
Instruments and techniques that require internal furnace access such as laser beam interferometry technology (See, B. Dahlberg and M. Brunner, Monitoring Lining Wear Through Laser Beam Technology, AGA IMS 1600, Iron and Steel Engineer, p.38, Nov. 1982) and a proposed diffuse reflectance spectroscopy method (L. Galoisy, G. Calas, and M. Maquet, Alumina Fused Cast Quartz Refractory Aging Monitored by Nickel Crystal Chemistry, J. Mater. Res., Vol. 6, p. 2434, 1991) are limited to firnaces that have visible access to view internal refractory lining surfaces. Such visible access is not possible in many furnaces to all the surfaces that need to be monitored and particularly so when they are in operation. In addition, optical viewing techniques are not robust in furnace environments which can have smoky and otherwise dirty atmospheres that can obscure views. Laser interferometry techniques are also very sensitive to vibrations and such sensitivity complicates use of this instrumentation in an industrial environment. Further, the reflectance spectroscopy method referred to above does not measure refractory thickness but only chemical changes in the surface of the refractory material.
Embedding wire loops and thermocouples into the refractory lining has also been used to determine refractory thickness. R. A. Strimple, C. R. Beechan, and J. F. Muhlhauser in Monitoring Brick Thickness and Hot-Face Temperature in EAF Sidewalls, Electric Furnace Conference Proc., Vol. 31, p. 214, 1974, describe a method utilizing wire loops spaced at 1 inch distances along the depth of the refractory and monitoring the loss of loop conductivity as the furnace lining wears through to break the loops. This method is limited in its spatial resolution of the lining thickness and in the number of points that can be monitored. The embedded wire loops may also be points of weakness at which the refractory lining might fail. M. Konishi, N. Nagai, T. Horiuchi, Y. Kawate, T. Uehara, K. Shimomura, and H. Sonoi in The On-Line Monitoring Method of Lining Erosion in Blast Furnace, Proc, of the 44th Ironmaking Conf., Iron and Steel Soc. of AIME, p. 511, 1985, have used methods that employ multiple thermocouples in the refractory lining from which thermal models are used to calculate refractory thicknesses. One thermal model relies on the time response of thermal fluctuations propagating through the refractory and another on the calculation of isothermals in the refractory. This approach is an indirect thickness measurement technique which can only be as accurate as the assumptions relied upon in the models.
Nuclear techniques have also been used and/or proposed for monitoring furnace lining wear. A. S. Prasad, P. Sinha, M. Qamrul, A. Chatterjee, P. K. Chakravarty, in Some Experience with Radio-Isotopes in the Study of the Wear of Blast Furnace Linings, TISCO(Tata Iron and Steel Co.) Vol. 26, p. 81, 1979, have used radioactive isotopes installed in the furnace lining and monitored the gamma radiation to study lining wear. Radioactively contaminating a furnace so as to monitor refractory corrosion is obviously of limited applicability. L. Staicu and I. Apostol in The Use of the (.lambda., n) Reaction for Checking Wear of Refractory Lining of Industrial Furnaces, Nuclear Instruments and Methods, Vol. 196, p. 511, 1982, have proposed a gamma-neutron reaction monitor in which the gamma source is safely contained but causes the emission of neutrons from a beryllium oxide sample attached to the exposed surface of the refractory. As the beryllium sample wears away, the neutron signal decreases. This method eliminates radioactive contamination of the furnace, but beryllium itself is a highly toxic metal which could not be readily used in many furnace installations.